Incidently, a Japanese company - Kujaku - does make a leaded enamel that gives a beautiful true ruby red straight on to silver. I don't know if it's available in the USA but would highly recommend trying it out.
Coral Shaffer at Enamelworks carrys a “red for silver” #105B that
gives a transparent ruby red directly on silver. I’m not sure if it
is Ninomiya color. She also carries a transparent pink that needs
no flux undercoat. Order from Coral–she’s very helpful and the
Ninomiya enamels are incredible.